This invention relates generally to a control system and method for a plurality of motors and more particularly, but not by way of limitation, to a pumping system and method wherein a plurality of motors are controlled to operate connected pumps that communicate with a reservoir of fluid.
An electric motor used to drive a device at a variable speed can be energized by a motor drive unit that converts the electrical power signal from a primary power source to a suitable level or frequency by which the speed of the motor, and thus of the driven device, is varied. When a maximum speed is reached, or when there is a problem with the motor drive unit, the motor may be switched directly to the primary power source, which typically is intended to provide a constant voltage and frequency. An example of a primary power source is an alternating current power network of an electric utility. "Directly" is used above to distinguish from the connection of the motor to a motor drive unit, but a "direct" connection as used herein can include other types of intervening devices, such as step-up and step-down transformers, fuses, circuit breakers, etc.
The motor drive unit referred to above is of the type that is used with a single motor; therefore, when several motors are needed, a like number of motor drive units are used. To reduce the expense of having a respective motor drive unit for each motor, there is the need for a system and method by which only one motor drive unit need be used with several motors. Despite having only one motor drive unit, such a system and method should be able to operate even if the one motor drive unit fails. Furthermore, it would be desirable for such a system and method to be able to switch motors to and from the motor drive unit without first stopping the motors to reduce the effects of sudden changes on the motors, devices driven by the motors and the systems in which they are used.
Such a system and method would be useful in many applications, a specific one of which is for driving pumps at a wastewater treatment plant. Sewage treatment is susceptible to "shock loads" which occur if the incoming flow rate varies over wide ranges; however, a treatment plant operates more efficiently if a smooth input flow rate is provided. Although a completely stable or constant flow rate cannot always be provided, one that is less susceptible to large fluctuations or shock loads can be provided from a variable motor-driven pumping system that tries to maintain a constant level of fluid in the wells or reservoirs accumulating the incoming flows prior to the accumulated wastewater entering the treatment process.
As another example, a system and method using a single motor drive unit would also be useful for driving fans on cooling towers or fans on large air conditioning systems where variable speed fan operation has been desired but has not been economically feasible because one variable speed drive unit would have been used for each fan motor. For example, putting enough individual variable speed motor drive units to control a 22-fan cooling tower of which we are aware could cost approximately $500,000.
Still other applications include, for example, any multiple motor driven centrifigal load devices such as centrifugal compressors, gear boxes, blowers, fans, pumps, etc. that can be direct coupled or belt driven, for example.
In the above example of a pumping system at a wastewater treatment plant, the flow of fluid into the reservoir is variable; therefore, the pumping system needs to provide for variable speed pumping, which requires variable speed motors such as the aforementioned type that are operated through motor drive units. Furthermore, the fluid reservoir is large enough that several pumps are sometimes needed; therefore, the pumping system preferably should keep track of the usage of the pumps and rotate them in variable utilization cycles to even out their time of operation.
Because motors within a single system, and the pumps driven by the motors in our specific example of a wastewater treatment plant, can have different characteristics, it also would be desirable to be able to control the motors (and thus their driven devices) in response to one or more such characteristics. For example, it would be desirable to operate a system of motor-driven pumps in response to the individual power curves of each pump because this would enable more efficient use of the electrical power needed to operate the system.
It would also be desirable for such a method and system as generally referred to above to have the capability of providing a less expensive form of electrical and/or hydraulic surge control when starting and stopping large pumps. To prevent hydraulic "hammering", for example, existing techniques use large, expensive surge control valves (e.g., a 450 horsepower pump surge valve can cost approximately $14,000 and weigh approximately 2,500 pounds).
Thus, there is the general need for a system and method by which only one motor drive unit need be used with several motors, and there are the additional needs within such system and method as exemplified above.